This invention pertains to the field of removing contaminants from a liquid, including, more specifically, removing heavy metals from industrial wastewater.
Many industrial processes produce wastewater streams that are laden with contaminants. These industrial processes include, among others, electroplating, galvanizing, anodizing, chelating, metal finishing, printed circuit board (PCB) manufacturing, semiconductor, magnetic disk manufacturing, mining operations, photo processing, fungicide manufacturing, food preparation, paper and pulp, textile, and oil refining. The wastewater streams of these different processes may contain any number of contaminants, including heavy metals, organic wastes, and inorganic wastes. In regard to heavy metal contaminants, they generally include, but are not limited to, metals such as copper, iron, gold, lead, nickel, silver, tin, zinc, chromium, cadmium, and arsenic.
The presence of these metals in wastewater causes the wastewater to be highly toxic. They can make the wastewater corrosive, inflammable, and even explosive. Due to the toxicity of metal laden wastewater, it poses a real danger of damaging wastewater collection systems, such as publicly owned treatment works (POTW), and of harming the environment.
In order to address the risks that metal laden wastewater presents, strict regulations have been imposed on plants regarding their wastewater discharges. Various agencies currently set maximum limits on the quantity of metals that plants may discharge into their waste streams. Where a plant discharges its wastewater to a POTW, these maximum limits are set either by the POTW itself, or by a municipal agency. And where a plant is discharging its wastewater directly to the environment, the maximum limits are typically set by state regulatory agencies and/or the Environmental Protection Agency.
Because of this need to minimize the quantity of metals discharged, plants treat their wastewater streams to remove the majority of the metals present. Since each metal has an optimum pH at which it will precipitate out of wastewater, plants have conventionally removed these metals individually using hydroxide precipitation over a series of pH adjustments, or by segregating waste streams that contain different metals and treating them individually. At each pH adjustment, at least one metal present in the wastewater will react with the treatment chemicals that have been added and will precipitate out of the wastewater. The metal precipitates must also be given a sufficient amount of time to settle out. The wastewater is then moved to another tank for the next pH adjustment. The wastewater must be moved to a new tank because once the pH level is changed, the metal that was just removed will have a tendency to re-dissolve back into solution.
For example, metals such as iron precipitate out of solution at lower pH levels, while metals such as nickel and cadmium precipitate out at higher pH levels. At a lower pH level, iron will precipitate out of solution, but if the pH level is then increased in order to remove other metals, the iron will dissolve back into solution. To solve this problem the wastewater is typically moved to a new treatment tank after a pH adjustment, leaving behind the metal that just precipitated out.
One drawback of known treatment processes is the length of time the precipitation of metals normally takes. Known methods chemically treat each metal separately, which requires many pH adjustments. In addition, the use of existing coagulants in known systems causes the metals to settle out slowly. Furthermore, known systems typically require a final pH adjustment prior to discharge. Thus the end result of all of these potential bottlenecks is that the entire operation may take anywhere from several hours to several days to complete.
Another drawback of known treatment systems is that when a plant generates several waste streams that each contain different metals, the waste streams are treated separately due to the problems involved in treating wastewater with multiple metals. This either forces a plant to implement more than one wastewater treatment system, or forces a plant to treat its waste streams one at a time. These limited options cause the plant to incur additional time and expense to treat all of its wastewater.
The fact that these processes can also be labor-intensive adds another source of time consumption. For example, plant operators often have to manually determine pH levels and manually add acid or base to adjust the pH levels, especially when spikes in metal concentrations occur. In addition, the chemicals that are added to the wastewater to precipitate out the metals can be in either solid or liquid form. This makes the addition of these chemicals into treatment tanks a more time-consuming process because operators typically add the solid chemicals manually, or have to initially mix the solid chemicals into clean water prior to adding it into the wastewater.
Another drawback to known systems is the fact that a plant""s treatment process normally has to be tailored to the specific composition of that individual plant""s wastewater so that it effectively removes the metals present. Generally, plants cannot simply implement an xe2x80x9coff-the-shelfxe2x80x9d process for treating their wastewater. Instead, plants typically have to design a treatment process around their effluent streams. This means that in the event of a system upset, for example higher levels of a metal or the introduction of a new metal in the wastewater, the treatment process will typically be less effective or ineffective altogether. The unfortunate result of this may be an unlawful discharge of metals. Thus, plants must continuously monitor the composition of their wastewater streams and modify the treatment processes and the chemicals they use to effectively treat their wastewater.
Other drawbacks of known systems relate. to flocculation and coagulation when known flocculants and coagulants are mixed into the wastewater. Coagulation is the process of combining the suspended metal solids, typically in the form of colloids or flocs, into larger and heavier particles. These larger particles become too heavy to remain suspended in the wastewater and drop to the bottom of the solution. A slightly different process that has similar results is flocculation, which is the process of physically trapping and/or linking the flocs together, typically through the use of a polymer. In known systems, one drawback is that most polymers are supplied in powder form, requiring the users to mix the powder into water prior to adding it into the wastewater. This is labor-intensive and time consuming process. Another drawback is that when flocculants are mixed into the wastewater, their flocculation effects are retarded by the mixing blades which tend to break-up the flocs that form. This results in sludge which is difficult to remove from the wastewater and from filters. In addition, the difficulty of removing sludge from filters is exacerbated by the fact that often, due to the use of high quantities of lime, the sludge is slimy and clings to filters, resulting in a high filter replacement rate.
Accordingly, there is a need for a process to remove metals from wastewater that is less time consuming and does not need to be specifically tailored for the wastewater composition of each plant in which it is used, and that also addresses the other drawbacks of known systems that were mentioned above.
The present invention addresses many of these aforementioned problems. The present invention is a process for treating wastewater that is faster than known methods, can be used on different compositions of wastewater without the need to individually tailor the process or chemicals to the specific composition of the wastewater, and produces a clear, virtually metal-free supernatant with a non-slimy sludge that has a high metal concentration. The system of the present invention is also easier to use and implement than known systems because only four chemicals are used in the system and all four of the chemicals are in liquid form.
The process of the present invention preferably comprises the following: measuring the flow rate of wastewater as it is fed into a first treatment tank; measuring the oxidation reduction potential of the wastewater; adjusting the pH of the wastewater to a level within a range of pH 9.3 to pH 9.5; mixing a first liquid elixir into the wastewater to react with any metal ions and/or chelates to form metal sulfates and/or less soluble metal complexes, and to bond with any metal sulfates created to prevent them from re-dissolving back into the wastewater, wherein the quantity of the first liquid elixir added to the wastewater is determined based at least in part upon the oxidation reduction potential of the wastewater and further based at least in part upon the flow rate of the wastewater; mixing a second liquid elixir into the wastewater to be treated to react with any metal sulfates and/or chelated metals to form insoluble metal hydroxides, and to bond with any metal hydroxides created to prevent them from re-dissolving back into the wastewater, wherein the quantity of the second liquid elixir added to the wastewater is determined based at least in part upon the quantity of the first liquid elixir added to the wastewater; feeding the wastewater into a second treatment tank; mixing a third liquid elixir into the wastewater to flocculate and coagulate the precipitates, wherein the quantity of the third liquid elixir added to the wastewater is determined based at least in part upon the quantity of the first liquid elixir added to the wastewater; mixing a fourth liquid elixir into the wastewater to flocculate and coagulate the precipitates, wherein the quantity of the fourth liquid elixir added to the wastewater is determined based at least in part upon the quantity of the first liquid elixir added to the wastewater; and separating the flocculated and coagulated precipitates from the wastewater.